Updated 20:30BST: British Airways has been forced to cancel all its flights leaving today from London airports Heathrow and Gatwick after experiencing a "major IT system failure."

The airline said that problem is also causing "very severe disruption to our flight operations worldwide". It said power supply problems, and not a cyber-attack, was the cause.

The airline had originally cancelled all flights from Heathrow and Gatwick before 6pm UK time today, Saturday 27 May as a result of the problems. But in an update at 16.45 BST it said that "with regret" it has had to cancel all flights leaving from Heathrow and Gatwick for the rest of today.

"We are working hard to get our customers who were due to fly today onto the next available flights over the course of the rest of the weekend. Those unable to fly will be offered a full refund," it said in a posting on Facebook.

The airline said the system outage has also affected its call centres and its website.

Speaking from the airline's operations centre near to Heathrow in a video posted on Twitter, British Airways CEO Alex Cruz that check-in and operational systems had been affected. And said the IT teams are working "tirelessly" to fix the problems.

"We believe the root cause was a power supply issue and we have no evidence of any cyber attack," he said.

British Airways said most long-haul flights due to land in London tomorrow (Sunday, May 28) are expected to arrive as normal. "We are working to restore our services from tomorrow, although some delays and disruption may continue into Sunday," the company said.

The IT problems coincide with the start of the half term holidays for a number of schools.

"We are extremely sorry for the inconvenience this is causing our customers and we are working to resolve the situation as quickly as possible," the company said in a post on Facebook.

On Twitter the company described the problems as a 'global outage' and said that it was unable to update its website due to the technical difficulties.